1. Field of the Invention
The present invention relates to a method for determining row correction values for a digital image converter having image points arranged in a matrix with rows and columns, in which a small part is protected against radiation by a covering in order to form a dark reference zone.
2. Description of the Prior Art
Digital imaging systems, in which digital image converters replace the previously used analog image converters, are becoming increasingly important in medical technology. In general, image converters of this sort that are used, e.g. a-Si detectors, still cannot be produced without errors. In addition to defective pixels or image points, group (cluster), row and column failures are still found. Thus, for example, column failures are caused by interruptions in the address lines.
In order to limit rejection of detectors, the defective image points can be corrected. For a successful correction, an identification of which pixels are defective and which are good is required.
In German PS 195 27 148, a method is specified for defect recognition in an a-Si panel in which, for the recognition of defective image points, a recursive low-pass filtering according to the unsharp or fuzzy mask principle, with separation of the defects according to rows and columns, is applied in multiple fashion to two images, of which a first image is produced with X-ray radiation without an object and a second image is produced without X-ray radiation.
At the left edge of the image, the beginning of the row in a radiography solid-state detector, e.g. an a-Si detector, there is a non-irradiated edge zone. This dark reference zone is covered in light-tight and radiation-tight fashion, so that even in the bright image no additional signal arises. The signal values of the unilluminated pixels are used for the correction of the row signal. They serve to reduce the row noise and to correct small transients in the offset, and again defects are taken into account so that they do not falsify the row correction.
For example, in the exposure of a pelvis, strongly over-radiated (bloomed) regions can occur between the legs. If these bloomed regions are located in the vicinity of the dark reference zone (DRZ), they nonetheless lead, e.g. by means of light conducted in the substrate or in the carrier material, to an irradiation of the dark reference zone, so-called sub-radiation. This results in a disturbing signal which, given the previous corrections, leads to disadvantages in the row correction in the form of fixed-pattern noise and butting problems. For example, caused by defects in the DRZ, a fixed-pattern row noise can arise above all given a sub-radiation of the DRZ. Moreover, due to the sub-radiation of the DRZ, row-corrected and uncorrected images can differ greatly. Also, given butting, a clearly visible separating line can even arise between two panels if sub-radiation has occurred on one of the panels.
These effects are caused by the insufficient treatment of the defects in the sub-radiated DRZ, and due to the fact that in sub-radiated regions in the DRZ a signal arises that increases strongly toward the image region. This is erroneously subtracted during the correction.
An object of the present invention is to provide a method of the type described above in which an optimal offset value is produced in each line for the reduction of the line noise.
The object is achieved according to the invention by averaging the differences of the image points in the dark reference zone in relation to their surrounding environment as a row correction value, wherein defective pixels in the DRZ are determined and are suppressed during the determination of the row correction value. By the averaging of the differences for the row correction, which takes into account a particular defect map determined previously on the basis of calibration and/or defect recognition, for each row an optimal correction value is obtained by means of which the row noise (fixed-pattern noise) is reduced.
It has proven advantageous to employ a specific high-pass filtering of the DRZ for the row correction so as to eliminate the low frequencies arising in the DRZ due to the sub-radiation before the row correction. According to the invention, the high-pass filtering of the DRZ can thereby take place in the column direction.
The error rate can be further reduced in an embodiment wherein, given a defect inside the column environment of the DRZ, the pixel located symmetrically with respect to the DRZ is also suppressed during the averaging in the column environment.
The inventive method can contain the following steps:
i. calculation of an average value of the environment of each pixel of the DRZ inside its column,
ii. formation of the differences of this pixel to the average values of its environment (high-pass filtering of the column) and
iii. formation of the average value of the differences as a correction value.
The determination of row correction values (ky) can take place according to the following equation:       k    y    =            1      l        ⁢                  ∑                  x          =          1                l            ⁢              xe2x80x83            ⁢              (                              p                          x              ⁢                              xe2x80x83                            ⁢              y                                -                                    1                                                2                  ⁢                  h                                +                1                                      ⁢                                          ∑                                  i                  =                                      y                    -                    h                                                                    y                  +                  h                                            ⁢                              xe2x80x83                            ⁢                              p                                  x                  ⁢                                      xe2x80x83                                    ⁢                  i                                                                    )            
with
Pxy=pixel column x, row y
l=length DRZ and
h=filtering level,
whereby the elements with defective pixels are left out of consideration.